Deregulation of kinase activity has emerged as a major mechanism by which cancer cells evade normal physiological constraints on growth and survival. To date, 11 kinase inhibitors have received US Food and Drug Administration approval as cancer treatments, and there are considerable efforts to develop selective small molecule inhibitors for a host of other kinases that are implicated in cancer and other diseases. Herein we discuss the current challenges in the field, such as designing selective inhibitors and developing strategies to overcome resistance mutations. This Review provides a broad overview of some of the approaches currently used to discover and characterize new kinase inhibitors.

The mammalian target of rapamycin (mTOR) kinase is the catalytic subunit of two functionally distinct complexes, mTORC1 and mTORC2, that coordinately promote cell growth, proliferation, and survival. Rapamycin is a potent allosteric mTORC1 inhibitor with clinical applications as an immunosuppressant and anti-cancer agent. Here we find that Torin1, a highly potent and selective ATP-competitive mTOR inhibitor that directly inhibits both complexes, impairs cell growth and proliferation to a far greater degree than rapamycin. Surprisingly, these effects are independent of mTORC2 inhibition and are instead because of suppression of rapamycin-resistant functions of mTORC1 that are necessary for cap-dependent translation and suppression of autophagy. These effects are at least partly mediated by mTORC1-dependent and rapamycin-resistant phosphorylation of 4E-BP1. Our findings challenge the assumption that rapamycin completely inhibits mTORC1 and indicate that direct inhibitors of mTORC1 kinase activity may be more successful than rapamycin at inhibiting tumors that depend on mTORC1.

Summary Embryonic stem cells (ESCs) of mice and humans have distinct molecular and biological characteristics, raising the question whether an earlier ‘naive’ state of pluripotency may exist in humans. Here we took a systematic approach to identify small molecules that support self-renewal of naive human ESCs based on maintenance of endogenous OCT4 distal enhancer activity, a molecular signature of ground state pluripotency. Iterative chemical screening identified a combination of five kinase inhibitors that induces and maintains OCT4 distal enhancer activity when applied directly to conventional human ESCs. These inhibitors generate human pluripotent cells in which transcription factors associated with the ground state of pluripotency are highly upregulated and bivalent chromatin domains are depleted. Comparison with previously reported naive human ESCs indicates that our conditions capture a distinct pluripotent state in humans that closely resembles mouse ESCs. This study presents a framework for defining the culture requirements of naive human pluripotent cells.

Neuroblastoma, an embryonal tumor of the peripheral sympathetic nervous system, accounts for approximately 15% of all deaths due to childhood cancer1. High-risk neuroblastomas, prevalent in the majority of patients, are rapidly progressive; even with intensive myeloablative chemotherapy, relapse is common and almost uniformly fatal2,3. Here we report the detection of previously unknown mutations in the ALK gene, which encodes a receptor tyrosine kinase, in 8% of primary neuroblastomas. Five non-synonymous sequence variations were identified in the kinase domain of ALK, of which three were somatic and two were germline. The most frequent mutation, F1174L, was also identified in three different neuroblastoma cell lines. ALK cDNAs encoding the F1174L and R1275Q variants, but not the wild-type ALK cDNA, transformed IL-3-dependent murine hematopoietic Ba/F3 cells to cytokine-independent growth. Ba/F3 cells expressing these mutations were sensitive to a small-molecule inhibitor of ALK, TAE6844. Furthermore, two human neuroblastoma cell lines harboring the F1174L mutation were sensitive to the inhibitor. Cytotoxicity was associated with increased levels of apoptosis as measured by TUNEL-labeling. shRNA-mediated knockdown of ALK expression in neuroblastoma cell lines with the F1174L mutation also resulted in apoptosis and impaired cell proliferation. Thus, activating alleles of the ALK receptor tyrosine kinase are present in primary neuroblastoma tumors and in established neuroblastoma cell lines, and confer sensitivity to ALK inhibition with small molecules, providing a molecular rationale for targeted therapy of this disease.

SUMMARY In an effort to find new pharmacological modalities to overcome resistance to ATP-site inhibitors of Bcr-Abl, we recently reported the discovery of GNF-2, a selective allosteric Bcr-Abl inhibitor. Here, using solution NMR, X-ray crystallography, mutagenesis and hydrogen exchange mass spectrometry we demonstrate that GNF-2 binds to the myristate binding site of Abl, leading to changes in the structural dynamics of the ATP-binding site. GNF-5, an analog of GNF-2 having improved pharmacokinetic properties, when utilized in combination with the ATP-competitive inhibitors imatinib or nilotinib, suppressed the emergence of resistance mutations in vitro, displayed additive inhibitory activity in biochemical and cellular assays against T315I Bcr-Abl and displayed in vivo efficacy against the recalcitrant T315I Bcr-Abl mutant in a murine bone-marrow transplantation model. These results demonstrate that therapeutically relevant inhibition of Bcr-Abl activity can be achieved using inhibitors that bind to the myristate binding site and that combining allosteric and ATP-competitive inhibitors can overcome resistance to either agent alone.

Genetic alterations that activate the mitogen-activated protein kinase (MAP kinase) pathway occur commonly in cancer. For example, the majority of melanomas harbor mutations in the BRAF oncogene, which are predicted to confer enhanced sensitivity to pharmacologic MAP kinase inhibition (e.g., RAF or MEK inhibitors). We investigated the clinical relevance of MEK dependency in melanoma by massively parallel sequencing of resistant clones generated from a MEK1 random mutagenesis screen in vitro, as well as tumors obtained from relapsed patients following treatment with AZD6244, an allosteric MEK inhibitor. Most mutations conferring resistance to MEK inhibition in vitro populated the allosteric drug binding pocket or ␣-helix C and showed robust (Ϸ100-fold) resistance to allosteric MEK inhibition. Other mutations affected MEK1 codons located within or abutting the Nterminal negative regulatory helix (helix A), which also undergo gain-of-function germline mutations in cardio-facio-cutaneous (CFC) syndrome. One such mutation, MEK1(P124L), was identified in a resistant metastatic focus that emerged in a melanoma patient treated with AZD6244. Both MEK1(P124L) and MEK1(Q56P), which disrupts helix A, conferred cross-resistance to PLX4720, a selective B-RAF inhibitor. However, exposing BRAF-mutant melanoma cells to AZD6244 and PLX4720 in combination prevented emergence of resistant clones. These results affirm the importance of MEK dependency in BRAF-mutant melanoma and suggest novel mechanisms of resistance to MEK and B-RAF inhibitors that may have important clinical implications.BRAF ͉ drug resistance ͉ MAP kinase ͉ melanoma A pproximately one-third of all cancers harbor genetic alterations that aberrantly upregulate mitogen-activated protein kinase (MAPK)-dependent signal transduction (1). In the MAPK pathway, RAS oncoproteins activate RAF, MEK, and ERK kinases to direct key cell proliferative and survival signals. When rendered constitutively active by genetic mutation, the MAP kinase pathway is believed to confer ''oncogene dependency'' (2), an excessive reliance on its dysregulated activity for tumor viability. Therefore, protein kinases within this signaling cascade offer promising targets for novel anticancer therapeutics.In melanoma, uncontrolled MAP kinase pathway activity is nearly ubiquitous and occurs most commonly through gain-offunction mutations involving codon 600 of the B-RAF kinase (3) (BRAF V600E ; 50-70% of cases). Considerable preclinical evidence has associated the BRAF V600E mutation with heightened sensitivity to pharmacologic inhibition of RAF or MEK kinases (4, 5). Although early clinical trials of RAF and MEK inhibitors failed to show a substantial benefit (6, 7), recent phase I studies of selective RAF inhibitors have shown promising results in patients with BRAF-mutant tumors (8, 9). Thus, optimizing therapeutic efficacy while avoiding or bypassing the emergence of resistance to MAP kinase pathway inhibition will likely gain increasing importance in melanoma and other MAP kinasedriven cancers.He...

While genomically targeted therapies have improved outcomes for patients with lung adenocarcinoma, little is known about the genomic alterations which drive squamous cell lung cancer. Sanger sequencing of the tyrosine kinome identified mutations in the DDR2 kinase gene in 3.8% of squamous cell lung cancers and cell lines. Squamous lung cancer cell lines harboring DDR2 mutations were selectively killed by knock-down of DDR2 by RNAi or by treatment with the multi-targeted kinase inhibitor dasatinib. Tumors established from a DDR2 mutant cell line were sensitive to dasatinib in xenograft models. Expression of mutated DDR2 led to cellular transformation which was blocked by dasatinib. A squamous cell lung cancer patient with a response to dasatinib and erlotinib treatment harbored a DDR2 kinase domain mutation. These data suggest that gain-of-function mutations in DDR2 are important oncogenic events and are amenable to therapy with dasatinib. As dasatinib is already approved for use, these findings could be rapidly translated into clinical trials.

Summary Protein kinases are intensely studied mediators of cellular signaling, yet important questions remain regarding their regulation and in vivo properties. Here we use a probe-based chemoprotemics platform to profile several well studied kinase inhibitors against more than 200 kinases in native cell proteomes and reveal new biological targets for some of these inhibitors. Several striking differences were identified between native and recombinant kinase inhibitory profiles, in particular, for the Raf kinases. The native kinase binding profiles presented here closely mirror the cellular activity of these inhibitors, even when the inhibition profiles differ dramatically from recombinant assay results. Additionally, Raf activation events could be detected upon live cell treatment with inhibitors. These studies highlight the complexities of protein kinase behavior in the cellular context and demonstrate that profiling with only recombinant/purified enzymes can be misleading.